Electric vehicles (“EV's”) and hybrid electric vehicles (“HEV's”) are becoming more viable alternatives to gas-powered vehicles, but present unique challenges not found in conventional gas-powered vehicles. Electric and hybrid vehicles typically have a battery pack that includes multiple cells in series to produce a high voltage that may be on the order of hundreds of volts. In conventional gas-powered vehicles, the vehicle starter battery has a much lower voltage, typically 12-14 volts.
In EVs and HEVs, high voltage circuitry may be needed to make high voltage measurements, such as for monitoring of the pack voltage. It is desirable to check for the occurrence of a fault between the high voltage terminal of the battery pack and chassis
High voltage (“HV”) measurements can be performed to verify the state of contactors and safely execute the contactor state machine, to command an appropriate charging voltage and verify the charging voltage, to verify and determine charge and discharge limits, which are based in part on pack voltage, and to validate the sum of cell voltage measurements (e.g., by comparing this sum to the measured pack voltage). IF HV measurements fail, there is the possibility that the vehicle could not properly close HV contactors to enable the drive or charge mode, or could not detect a failed contactor that is stuck open or closed. There is also the possibility that the battery pack could be provided with the incorrect charge voltage or fail to charge, or that cell voltage checks would fail, preventing the vehicle from being driven.
As used herein, high voltage measurements are considered to be measurements of the pack voltage of a battery, or of systems connected to the pack voltage. Pack voltages can range from tens of volts (e.g., 20 V, 30 V, 50 V, etc.) to a few thousand volts. High Voltage (HV) measurements are not needed for measurement of the starter battery, or measurement of individual cells making up the battery pack. If the battery pack is split up into a small number of sections, such as a positive half and a negative half separated by a mid-pack disconnect, measurement of the sections would be included as high voltage measurements.
Throughout this document, the terms battery pack and battery stack may be used interchangeably.
Accurate measurement of high voltage is imperative in any battery system that contains such voltages. To provide a safe and robust system, contactors may be used to provide and remove high voltage from the service accessible terminals. A “contactor” is an electrically controlled switch used for switching a power circuit, and similar to a relay except with higher current and/or ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit.
The ability to measure high voltage to verify the proper operation of these contactors can be provided. Accurate high voltage measurements provide feedback to the battery management system as to the state of these contactors.
The battery system will interface with external chargers. The high voltage measurement needs to be accurately provided to these chargers to ensure that an appropriate charging voltage is provided. It is also typically the responsibility of the battery management system to verify the voltage coming from the charger.
A battery management system needs to maintain accurate information about the battery stack in order to make intelligent decisions about the system. These decisions include, but are not limited to, the use of precharging systems and deciding when to close contactors, providing current and voltage limits to systems that are connected to the battery pack such as chargers and motor controllers, and data collection. Additionally, the pack high voltage measurement is used to validate the cell voltage measurements by comparing the pack HV measurement to the sum of the cell measurements. This provides an additional validation that the cell voltage measurements are accurate.